There has been a long-standing need for mapping extended sources for radioactive "hot spots." Increasing industrial and military use of plutonium and other alpha particle emitters has most definitely exacerbated the risk of personnel contamination. In particular, radioactive materials can enter wounds resulting from accidents. Such wounds require immediate detection and treatment. At present, portable, reasonable resolution detectors of radiation which could be used to examine wounds and localize contaminated areas are gamma radiation detectors, there being no equivalent device for alpha particles. As such, these devices are useless in evaluating plutonium contamination because plutonium is a strong alpha particle emitter, but only a weak gamma ray source. Although sensitive alpha particle detectors exist, they are large, nonportable and have poor spatial resolution. They provide a simple yes-no answer to the question of decontamination necessity. A positive finding could lead to time consuming, unpleasant, and perhaps unnecessary cleansing procedures when a localized, more effective one would be advantageous. That is, it might appear from a whole body count that a person was "hot" enough to require general decontamination when in actuallty, a small, highly radioactive area such as a wound or under fingernails demands immediate and specialized treatment. Indeed, the value of a sensitive, high spatial resolution device is apparent for emergency situations where time is of the essence.
The instant invention is a self-triggering spark chamber and method for alpha particle monitoring and imaging. My device can be used, among other things, for scanning a contaminated individual directly to quickly localize the affected area even in a fully lighted room in order that accurate, quantitative decisions can be made concerning the approach to meeting the crisis. Although the device of this invention describes a hand-held viewing screen in a two electrode configuration into which the charged particles pass through a thin window rather than the radiation source being placed in the chamber gas, the device can be enlarged to enable viewing of large portions of the subject under investigation with no loss in sensitivity or resolution. Radiation can also be electronically counted or integrated over time as a photograph. Alpha particles are detected from the electrical discharges they produce upon entering the spark chamber. Such particles are generally sufficiently energetic that they can dislodge electrons from atoms or molecules into which they collide. Many such collisions occur before the alpha particle is slowed sufficiently so as to be ineffective in producing additional free electrons. Under the influence of the applied electrostatic field, the freed or unbound electrons are accelerated producing more electrons as they collide with atoms and molecules in their way during their journey to the more positive electrode. This so-called "avalanche" causes a visible spark to appear. Such sparks can be easily observed with the unaided eye or detected using electronic counting methods. The background events measured by the instant invention are low because the device doesn't detect either gamma rays or beta particles with significant efficiency. Therefore, the specificity and sensitivity for alpha emissions is high, and the device serves well as a plutonium detector. Among other uses for my instrument are the determination of the uniformity of planar alpha particle emitters used for radiation effect investigations, and the search for radioactive "hot spots" on air filters to decide whether the contamination is a fine dust or a single particle. Dust implies greater danger to personnel while the particle may be spurious.
The present invention relates generally to detection or alpha particles and more particularly to a portable, self-triggering, imaging spark chamber. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Four relevant references should be discussed to adequately place the present invention with respect to existing art. It should be mentioned that three out of the four references teach gamma radiation and beta particle detection, neither emission being sufficiently closely related to alpha particles in their ionization characteristics to be illustrative in the design of the instant invention.
1. "Spark Chambers in Nuclear Medicine," by A. J. Lansiart and C. Kellershohn, Nucleonics 24, 56 (1966), describe a xenon filled, self-triggering spark chamber for use in detecting gamma rays in nuclear medicine investigations. The device would require major redesigning to be useful for alpha particle detection which redesigning the article does not teach. First, the lead collimator and aluminum cathode would have to be removed and replaced by a window material which simultaneously transmits alpha particles and can maintain the necessary gas-tight characteristics. Second, and most important, the sizable detection gap which is used to give some gain or multiplication to the charge deposited by the gamma radiation in the xenon gas mixed with a small amount of methylal would simply result in a "smeared-out" and therefore useless profile of the emitting source. That is, alpha particles ionize significant numbers of atoms or molecules in gases they come in contact with, whereas gamma radiation and beta particles are nowhere near as efficient in producing charge particles upon collision with neutral species. As a result, the additional path for amplification of charge provided in the Lansiart and Kellershohn device would produce a broad avalanche of electrons at every point along the path of the incident alpha particles thereby destroying the desired resolution of an imaging device. Finally, the referenced device teaches a triode (cathode, grid, anode) configuration while for our invention, a simpler diode design (grid, anode) is quite adequate because of the ionization efficiency of the alpha particles. The instant invention is therefore not suggested by the Lansiart and Kellershohn article.
2. "A Hybrid Spark Chamber for Measuring Radionuclide Distributions," by Takahiko Aoyama and Tamaki Watanabe, Nuclear Instruments and Methods 150, 203 (1978), again teaches away from the diode design of our invention by describing a beta particle detector. The window transmission problem is solved by putting the radiation source inside a sealed envelope which is flushed with argon saturated with ethanol. This procedure, of course, renders the apparatus virtually useless for the emergency applications planned and demonstrated for the instant device. Moreover, the gap between the cathode grid and the ground plane grid, which is critical for amplifying the charge deposited in the gas by the beta particles in the triode arrangement, as described supra, destroys the desired resolution of the apparatus when used to detect ionizing alpha particles. It is specifically mentioned in the reference that at the high voltages needed to efficiently detect beta particles, a diode configuration is unstable; hence the move to a triode design, and away from our invention. Finally, although the reference explicitly mentions expansion of the device area to about 400 cm.sup.2, the instrument described is designed to detect beta particles and would require major redesign, which is not taught in the reference, to detect alpha particles with the sensitivity and spatial resolution of the instant invention.
3. "A Portable X-Ray Imaging System for Small-Format Applications," by Lo I. Yin, Jacob I. Trombka and Stephen M. Seltzer, Nuclear Instruments and Methods 158, 175-180 (1979), teaches a compact, portable gamma radiation detector. However, the device cannot detect alpha particles without modification, again not taught by the reference. Further, the apparatus does not utilize the spark chamber design taught by the above two references and the instant invention, the price which one pays being the restriction of the viewing area. Finally, the device cannot be used unless the background lighting is subdued. This is in contrast with the instant invention which can be used in a lighted room.
4. "Selecting Spark Based on the Difference of Specific Ionization of High Energy Charged Particles Incident on a Self-Triggering Spark Chamber," by Takahiko Aoyama, Kazuaki Kamata, Yoshimitsu Kobayashi and Tamaki Watanabe, Radioisotopes 24, 305 (1975), is an article by two of the authors of Ref. 2, supra. A source of either beta or alpha particles is placed inside the vacuum envelope of the apparatus and sparks are observed under appropriate voltage conditions. A diode configuration is taught and selective detection of either betas or alphas can be achieved by simply adjusting the applied voltage. Reference 4 is written in Japanese, but a careful reading of the English abstract and the figures included in the article leads us to the following conclusions. Reference 2, supra, mentions that the detector characteristics of the earlier device (Ref. 4) are good for alpha particles, but must be modified for improvement in stability and sensitivity when used for beta particles. However, the placement of the radiation source inside the envelope does not allow use of the device for wound monitoring, as does our invention, and no mention is made of the resolution of the image obtained. Reference 4 appears therefore to be a study of the voltage characteristics of the type of device without any consideration of its use as a practical radiation detector, while Ref. 2 carries Ref. 4 to a more practical point by considering the resolution of a triode device. Neither teaches the use of a specifically chosen window material to allow passage of alpha particles from an external source and the resulting visual inspection of the emitting source, as does the instant invention. Neither teaches the use of a small electrode spacing such that only alpha particles are detected without the presence of gamma or beta radiation background counts.